Waning-and-waxing shape changes in ionic nanoplates upon cation exchange

Flexible control of the composition and morphology of nanocrystals (NCs) over a wide range is an essential technology for the creation of functional nanomaterials. Cation exchange (CE) is a facile method by which to finely tune the compositions of ionic NCs, providing an opportunity to obtain complex nanostructures that are difficult to form using conventional chemical synthesis procedures. However, due to their robust anion frameworks, CE cannot typically be used to modify the original morphology of the host NCs. In this study, we report an anisotropic morphological transformation of Cu1.8S NCs during CE. Upon partial CE of Cu1.8S nanoplates (NPLs) with Mn2+, the hexagonal NPLs are transformed into crescent-shaped Cu1.8S–MnS NPLs. Upon further CE, these crescent-shaped NPLs evolve back into completely hexagonal MnS NPLs. Comprehensive characterization of the intermediates reveals that this waxing-and-waning shape-evolution process is due to dissolution, redeposition, and intraparticle migration of Cu+ and S2−. Furthermore, in addition to Mn2+, this CE-induced transformation process occurs with Zn2+, Cd2+ and Fe3+. This finding presents a strategy by which to create heterostructured NCs with various morphologies and compositions under mild conditions.

1.In Fig. 1m, it is assumed that the atomic ratio of S should remain constant regardless of the ZnCl2/Cu1.8Sratio, as the dissolved S during the waning process grows into the MnS shell.What could be the cause of the changes in the S ratio?Similarly, in Fig. 4e, f, g, what causes the S ratio to change according to the reaction time?I am curious about the extent of noise influence during the elemental quantification analysis using EDX, and I wonder how much difference there is compared to the results from the inductively coupled plasma measurement.
2. Based on the hard and soft acid and base (HSAB) theory, I believe that the decomposition of Cu1.8S could vary based on the base ligands used.If halides other than chloride are employed, or if the ratio of oleylamine to trioctylphosphine is adjusted, would it be possible to suppress the waning phenomenon even in the occurrence of a partial exchange?3.There are errors in the figure numbers within the text.In the sentence on line 230, "Fig.4" should be corrected to "Fig.5", and in the sentence on line 251, "Supplementary Fig. 9" should be corrected to "Supplementary Fig. 10".
Reviewer #3 (Remarks to the Author): I co-reviewed this manuscript with one of the reviewers who provided the listed reports.This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts.
Reviewer #4 (Remarks to the Author): Post-synthetic nanoscale cation exchange reactions have emerged as a powerful synthesis strategy to circumvent thermodynamic and kinetic limitations, providing pathways to nanocrystals and heteronanocrystals with compositions, morphologies, crystal structures, and hetero-architectures that would otherwise remain unavailable.These prospects have turned the study of nanoscale cation exchange reactions into a very relevant scientific endeavour, which has been attracting increasing interest in recent years.Cation exchange reactions are typically topotactic, thereby preserving the overall morphology and structure of the anionic sublattice of the parent nanocrystal into the product nanocrystal.There are however several recent examples of cation exchange reactions that are accompanied by morphological transformations.
In this contribution, Teranishi and coworkers report an intriguing and unprecedented example of severe morphological reorganization in response to a cation exchange rection.The authors observe that upon partial aliovalent Mn2+ for Cu+ cation exchange, hexagonal roxbyite (rb) Cu1.8S nanoplates gradually transform into crescent-shaped Cu1.8S-MnS core-shell heteronanoplates, which upon further cation exchange evolve back into complete hexagonal nanoplates of wurtzite (wz) MnS.Similar cation exchange-induced transformations are also observed with Zn2+, Cd2+, and Fe3+.
The work has been carried out very thoroughly and provides novel and significant insights that make it of high interest to a broad scientific community.Therefore, I think it merits publication in Nature Communications.However, there are several issues that should be addressed prior to its publication.
My main concern regards the mechanism proposed for the transformation, which is not clearly and convincingly presented in the paper.Moreover, the contextualization of the work with the most current literature in the field could be improved.I will address my concerns in more detail below.
1. On page 3 (Figure 1) the authors demonstrate that under excess Mn2+ the cation exchange reaction is clearly topotactic, maintaining the original shape and dimensions of the parent rb Cu1.8S nanoplates into the product wz MnS nanoplates (apart from a small expansion due to the slightly larger lattice volumes of wz-MnS).The authors imply in the proposed mechanism that also in this case the reaction proceeds through a 'Waning and Waxing' cycle.However, this is not necessarily the case, given that if the CE reaction is under kinetic control, it could proceed sufficiently fast to prevent the "waning" phase.Moreover, the data presented is insufficient to support this interpretation since intermediate samples were not collected under excess Mn2+ conditions, only under sub-stoichiometric conditions, which may favor waning for the reasons mentioned below.
2. The discussion concerning Figures 2 and 3 attributes the formation of the crescent-shaped Cu1.8S-MnS core-shell heteronanoplates to cation exchange.The authors suggest that Mn2+ for Cu+ cation exchange in the parent Cu1.8SNPLs begins from one side while S2− is supplied from the opposite side through intraparticle migration.The dissolution of one half of the nanoplates is thus attributed to an imbalance between the rapid outward diffusion of Cu+ and the slower inward diffusion of Mn2+.
However, this mechanism is inconsistent with the data presented because the MnS shell is observed to grow heteroepitaxially over one half of the parent Cu1.8SNPLs, thereby making it thicker, while the other half gradually shrinks from the surface inwards.It has been shown for many different systems that core-shell heteronanocrystals formed by cation exchange reactions are characterized by superseded shell ingrowth, i.e., the total dimensions of the template nanocrystals are preserved while the shell grows thicker and the core shrinks due to the inwards diffusion flux of the guest cation and outward diffusion flux of the host cations (e.g., ZnSe-CdSe core-shell QDs [Groeneveld, E.;et al. ACS Nano (2013), 7, 7913], PbSe-CdSe core-shell QDs [Pietryga, J. M.;et al. J. Am. Chem. Soc. (2008), 130, 4879;Grodzinska, D.;et al., J. Mater. Chem. 2011, 21, 11556].The observation reported in the present contribution is reminiscent of that recently reported by Xia, C. et al. [ACS Nano 15 (2021) 9987] concerning the formation mechanism of colloidal Janus-Type Cu2-xS/CuInS2 Heteronanorods via Seeded-Injection, which is shown to start with a partial aliovalent In3+ for Cu+ cation exchange in just one facet of the Cu2-xS seed nanocrystals, converting it to wz-CuInS2.As soon as this wz-CuInS2 surface is formed, homoepitaxial growth of wz-CuInS2 takes over, outcompeting the cation exchange reaction for the limited In3+ supply and gradually consuming the Cu2-xS segment of the nanocrystals as sacrificial Cu+ sources.The formation of the Cu1.8S-MnS core-shell heteronanoplates seems to proceeds through a similar mechanism: Mn2+ for Cu+ cation exchange initially converts the bottom and top facets of the parent Cu1.8SNPLs into wz-MnS, thus triggering a fast homoepitaxial overgrowth of a MnS shell using the available Mn2+ in solution and the other half of the NPLs as sacrificial sulfur sources.CE only resumes when the supply of S2-has been exhausted (i.e., one half of the NPL has been entirely dissolved while the other is protected by the MnS shell) and proceeds inwards from the exposed side facets.The process would thus be triggered by a kinetic imbalance between the CE rates (too slow due to the low concentration of Mn2+) and the dissolution of the Cu1.8SNPLs by Cl-and homoepitaxial growth of MnS (both fast and kinetically coupled).
3. On page 10 the authors state that anisotropic CE progression and Cu1.8S etching by Cl-occur simultaneously to induce the transformation of hexagonal Cu1.8S NPLs into crescent-shaped Cu1.8S-MnS HNPLs.However, as discussed above this is inconsistent with the data presented and can be better explained by a kinetic imbalance.
4. On page 11 the authors invoke the nanoscale Kirkendall effect to explain the formation of the crescent-shaped Cu1.8S-MnS HNPLs (i.e., the "waning" phase).However, this seems unlikely as the Kirkendall effect should rather result in the formation of hollow NPLs (i.e., dissolution of the center of the NPLs), as previously reported by Buhro and coworkers for CuInS2 NCs (ref. [18]).To support this interpretation, the authors should also propose a reasonable explanation for the asymmetry observed in the process: why does one half of the NPL dissolves, while a MnS shell overgrows on the other?It seems more likely to me that the dissolution is driven by a kinetic imbalance in between etching rates by Cl-and the CE rates (similarly to the formation of hollow Cu2-xS nanocrystals through etching induced by GaCl3 [Hinterding, et al.;ACS Nano (2019), 13, 12880]).
5. The authors convincingly demonstrate that chloride is responsible for the etching and dissolution observed during the waning phase.However, metal chlorides are widely used to deliver guest cations in cation exchange reactions and do not seem to induce any etching.Why do they act as etching agents under the conditions used in the present work?What is different with respect to the conditions typically used in other works?6.The discussion on page 11 and elsewhere in the paper overlooks the importance of the solid-state diffusion rates of the guest cations in determining the overall heteroarchitecture of the product nanocrystals obtained by cation exchange reactions.This is a crucial point because it depends on the nature of both the guest cation and the host nanocrystal and on the temperature.Slower migration of the incoming guest cation does not necessarily result in collapse of the anionic sublattice and dissolution of the NC, but rather in the formation of heteronanocrystals with well-defined heterointerfaces, while fast diffusion results in homogeneous alloys.This has been extensively investigated by many groups, e.g., , Swihart andcoworkers [Chem. Mater. (2018), 30, 1399], Robinson and coworkers [Nano Lett. (2014), 14, 7090], Donega and coworkers [Groeneveld, E.;et al. ACS Nano (2013), 7, 7913;Hinterding, et al.;ACS Nano (2019), 13, 12880], Manna and coworkers [ref. 4]. 7. Page 12, line 230: "Fig.4" should be "Fig.5". 8.The mechanism proposed for the "waxing" process is rather vague and does not seem to be fully supported by the data presented.The cartoon in Figure 5 assumes that the starting configuration for the waxing process corresponds to the end configuration of the waning process.This is however not supported by the TEM images and STEM-EDX maps presented in Figure 5, which are consistent with the intermediate configuration shown in the cartoon in panel l of figure 5 but provide no evidence for the starting configuration.Moreover, the mechanism proposed leaves many intriguing questions without satisfactory answers, e.g.: (a) why would the MnS shell formed during the waning phase suddenly redissolve and regrow to regenerate the missing half of the nanoplate?The conditions are clearly favorable to MnS growth, so why would they at the same time promote the dissolution of what was seemingly a very stable MnS shell?(b) And why would the thickness of the NPL revert back to the original thickness?What prevents the CE to proceed and convert the Cu1.8S core into MnS while additional MnS grow on the sides of NPLs concomitantly reconstructs the missing half of the nanoplate, yielding a thicker hexagonal NPL? (c) Why does the Cu1.8S that was stored in the core of the core-shell Cu1.8S-MnS heteroNPLs suddenly migrates to the surface and spreads itself over just one of the wide facets?The authors are either missing several crucial intermediate steps in the conversion of the left cartoon to the middle cartoon or are mistaken in their mechanism proposed for the waxing process.9.In the final section of the paper the authors discuss their experiments with nanorods.How do their observations differe from what has already been extensively reported by Schaak and coworkers? (e.g.,) 10.Claim of universality at the end of paper is likely exaggerated.The authors investigated four different cations: Mn2+, Zn2+, Cd2+ and Fe3+.The waning-waxing cycle process occurs only for Mn2+ and Zn2+.Cd2+ gives Janus-type hetero-NPLs and Fe3+ gives homogeneous CuFeS2.What determines the different outcomes?Likely the different solid-state diffusion rates of the different cations, but this is an aspect that is largely overlooked in the mechanism proposed in the paper.
formation of Janus-like structure or multi-patchy structure) and etching of Cu1.8S region with TOP and O2.

Manuscript revision:
In the revised manuscript, we additionally cited the above 4 papers as refs 26, 27, 28 and 29, respectively.
Q2: All of the composition measurements seem to be based on EDX and XRF.Because these are important to the manuscript's conclusions, it would be nice if at least a fraction of the EDX-based composition measurements could be benchmarked against a more quantitative and truly bulk method, such as ICP-OES or ICP-MS.The EDX measurements are only as good as the calibration of the instrument, and most of us use the standard calibration from the manufacturer, which may not match the local environment and can thus bias the samples.This is not essential, because the trends will clearly be the same, even if the quantification is slightly off.
Author reply: Thank you for your suggestion.First, we tried ICP-OES analysis of the cation exchanged products, but the decomposition of metal sulfide nanocrystals by strong acid treatment during ICP sample preparation led to the volatilization of sulfur as H2S gas from the solution, which results in a lower estimation of S. To assess the accuracy of EDX measurements in determining Cu:Mn:S mole ratios, we prepared the mixed aqueous solutions from standard aqueous solutions for ICP analysis containing fixed concentration of Cu, Mn, or S (1000±10 mg/L for Cu and Mn, 1000±20 mg/L for S, purchased from FUJIFILM Wako).The mixed solutions were dried to obtain the solids containing Cu, Mn, and S for EDX measurements.Three distinct samples were prepared with different Cu:Mn:S ratios, so as to replicate samples with different degrees of cation exchange (CE) progression (Table R1).The results revealed that the mole ratios estimated from EDX analysis were in good agreement with the calculated values (Fig. R1).The Cu/Mn mole ratios also closely match the theoretical values, indicating the suitability of EDX analysis in estimating the degree of CE progression of Cu1.8S with Mn 2+ .Consequently, we would like to assert that EDX measurements give accurate composition values of cation-exchanged products in our experiments.
Table R1.Amounts of ICP standard solutions for sample preparation, calculated mole ratios, and mole ratios estimated by EDX measurements.Five distinct areas were measured for EDX analysis to obtain the average values and standard deviations.Overall, despite some precedents in the literature, I believe that the clear and thorough observation and presentation of this waxing-waning phenomenon during cation exchange merits publication in a high impact journal.
We appreciate your positive recommendation.

Author replies to the comments of Reviewer 2
The manuscript demonstrates the waning and waxing-like shape transitions of Cu1.8S nanocrystals during the cation exchange process.The authors meticulously analyzed the mechanism behind these transitions, observing the crystallographic changes in the nanocrystals using techniques such as transmission electron microscopy and X-ray diffraction.These intriguing results challenge the conventional perspective that the collapse of the anion framework during the cation exchange process irreversibly prevents the recovery of the template nanocrystals' structure.These findings will pave the way for novel methodologies in cation exchange-driven nanocrystal synthesis.I believe the manuscript would be suitable for publication in Nature Communications, provided that some revisions are made, including responses to below comments.
Author reply: We are grateful for your positive comments and constructive suggestions to improve our manuscript.
Q1: In Fig. 1m, it is assumed that the atomic ratio of S should remain constant regardless of the MnCl2/Cu1.8Sratio, as the dissolved S during the waning process grows into the MnS shell.What could be the cause of the changes in the S ratio?Similarly, in Fig. 4e, f, g, what causes the S ratio to change according to the reaction time?I am curious about the extent of noise influence during the elemental quantification analysis using EDX, and I wonder how much difference there is compared to the results from the inductively coupled plasma measurement.
Author reply: Thank you for your comments.According to the chemical formula, the mole ratios of S in Cu1.8S and in MS (M = Mn, Zn, Cd) should be 36% and 50%, respectively.Therefore, the mole ratio of S in the product should increase from 36% to 50% as the M 2+ cation exchange (CE) of Cu1.8S proceeds.This trend is consistent with our EDX results in Fig. 1m (and other EDX results).
The change in the mole ratios of S in Fig. 4e-g can be explained by the same story, while the mole ratios of S plateau out at <50% due to the formation of Cu1.8S-MS via partial CE reactions with insufficient M 2+ cations.
Figure R2 shows representative raw EDX spectra of Cu1.8S nanoplates (NPLs) before and after CE reactions.Long accumulation time gave smooth spectrum and high peak/background intensity ratio, allowing to calculate each mole ratio with small uncertainty.The difference in mole ratios with and without background subtraction is only approximately 0.5% for each element, also indicating the strong peak intensity (Table R2) (we use background-subtracted value in the manuscript).Table R2.Comparison of mole ratios calculated from EDX spectra in Fig. R2 with and without background subtraction.
Then, we investigated the difference between the EDX and ICP results.First, we tried ICP-OES analysis of the cation exchanged products, but the decomposition of metal sulfide nanocrystals by strong acid treatment during ICP sample preparation led to the volatilization of sulfur as H2S gas from the solution, which results in a lower estimation of S. To assess the accuracy of EDX measurements in determining Cu:Mn:S mole ratios, we prepared the mixed aqueous solutions from standard aqueous solutions for ICP analysis containing fixed concentration of Cu, Mn, or S (1000±10 mg/L for Cu and Mn, 1000±20 mg/L for S, purchased from FUJIFILM Wako).The mixed solutions were dried to obtain the solids containing Cu, Mn, and S for EDX measurements.Three distinct samples were prepared with different Cu:Mn:S ratios, so as to replicate samples with different degrees of cation exchange (CE) progression (Table R3).The results revealed that the mole ratios estimated from EDX analysis were in good agreement with the calculated values (Fig. R3).The Cu/Mn mole ratios also closely match the theoretical values, indicating the suitability of EDX analysis in estimating the degree of CE progression of Cu1.8S with Mn 2+ .Consequently, we would like to assert that EDX measurements give accurate composition values of cation-exchanged products in our experiments.
Table R3.Amounts of ICP standard solutions for sample preparation, calculated mole ratios, and mole ratios estimated by EDX measurements.Five distinct areas were measured for EDX analysis to obtain the average values and standard deviations.Q2: Based on the hard and soft acid and base (HSAB) theory, I believe that the decomposition of Cu1.8S could vary based on the base ligands used.If halides other than chloride are employed, or if the ratio of oleylamine to trioctylphosphine is adjusted, would it be possible to suppress the waning phenomenon even in the occurrence of a partial exchange?
Author reply: Thank you for your valuable suggestion.We used other Mn halides, MnBr2 and MnI2, as Mn precursors for CE reactions.MnF2 was not used because MnF2 was not dissolved in oleylamine/octadecene solvent.For both cases, the waning and waxing phenomenon took place, as observed in the case using MnCl2 (Fig. R4).From the viewpoint of the HSAB theory, all halides used here can be regarded as soft bases that have strong coordination ability with soft Cu + to cause etching of Cu1.8S (Table R4).Therefore, similar products were obtained regardless of halide anions.Table R4.The hardness of halide anions, trioctylphosphine (TOP), and oleylamine as Lewis bases, and Cu + and Mn 2+ as Lewis acids.(Refs: Pearson et al., Inorg. Chem. 1988, 27, 734-740;Alivisatos et al., J. Phys. Chem. C 2013, 117, 19759−19770.)Then, we conducted experiments employing various TOP concentrations.Typically, we used 3 mL of TOP for injection to synthesize Cu1.8S NPLs.The TOP quantity was varied by diluting the injection solution with octadecene, while maintaining the total volume at 3 mL (Table R5).The Mn content in the resulting products decreased with decreasing the TOP quantity.When TOP was reduced to 0.3 mL or 1 mL (1/10 or 1/3 of the typical amount, respectively) with 0.5 equivalent of MnCl2 to Cu1.8S, the Mn/Cu mole ratio in the products was 0.08 or 0.17, respectively (Entries 1 and 2 in Table R5).These ratios are notably lower than that when using 3 mL of TOP (0.69, Entry 4 in Table R5), which is in good agreement with the hypothesis that TOP induces CE by extracting Cu + from the host Cu1.8S NCs.
In comparison with the case using 3 mL of TOP, the morphological transformation was less obvious even with a similar Mn/Cu mole ratio in the products (Fig. R5).For instance, when using 3 mL of TOP, one sides of the NPLs initiated dissolution at a Mn/Cu mole ratio of 0.06 (Fig. R5d), while no evident dissolution was observed at a Mn/Cu mole ratio of 0.08 in the case using 0.3 mL of TOP (Fig. R5a).In the case using 1 mL of TOP, one sides of the NPLs slightly dissolved at a Mn/Cu mole ratio of 0.17 (Fig. R5b), while half of the NPLs underwent severe decomposition even at a Mn/Cu mole ratio of 0.13 using 3 mL of TOP (Fig. R5e).These different degrees of transformation can be explained by the role of TOP as an etching reagent, as reported in the literature, indicating that TOP dissolves Cu2−xS NCs by forming phosphine sulfide species [A. Nelson et al., Chem. Mater. 2016, 28, 8530. (ref. 35)].In our case, not only halide anions but also TOP quantity promoted the transformation of NPLs by hastening the dissolution of the Cu1.8SNPLs, resulting in a less degree of transformation when using smaller TOP quantities.Nevertheless, a subtle anisotropic change in the shape of NPLs could be observed when using 1 mL of TOP (as indicated by arrows in Fig. R5b), suggesting that the waning process initiates even at a lower TOP concentration.To observe further transformation with 1 mL of TOP, a larger quantity of MnCl2 was employed to enhance Mn 2+ CE (MnCl2/Cu1.8S= 1, Entry 3 in Table R5).As expected, the Mn/Cu mole ratio in the CE product increased to 0.29, and one sides of the NPLs underwent severe dissolution (Fig. R5c).These results strongly indicate that the anisotropic CE proceeds to give anisotropic shapes, even when the extracted quantity of Cu + is reduced by decreasing the TOP quantity.It is also suggested that, even at low CE ratios, shape changes may occur due to the intraparticle migration of cations if there is a significant difference in cation diffusion rates (Mn 2+ vs. Cu + ).
In summary, even when different halide salts and smaller TOP quantities were employed, the anisotropic shape changes proceeded, following the waning process.This is consistent with the explanation that the shape changes arise from differences in cation diffusion rates during CE, suggesting that this phenomenon is unique to the Cu + and Mn 2+ combination.
Table R5.Reaction conditions and EDX results of the products obtained by using different TOP quantities in CE. *Entry 4 corresponds to the result in the manuscript (Fig. 1m).Q3: There are errors in the figure numbers within the text.In the sentence on line 230, "Fig.4" should be corrected to "Fig.5", and in the sentence on line 251, "Supplementary Fig. 9" should be corrected to "Supplementary Fig. 10".
Author reply: Thank you for your careful reading.

Manuscript revision:
We revised the Figure numbers on pages 13 and 14.
I co-reviewed this manuscript with one of the reviewers who provided the listed reports.This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts.
Author reply: Thank you for reviewing our manuscript.We would like you to review our revised manuscript.

Author replies to the comments of Reviewer 4
Post-synthetic nanoscale cation exchange reactions have emerged as a powerful synthesis strategy to circumvent thermodynamic and kinetic limitations, providing pathways to nanocrystals and hetero-nanocrystals with compositions, morphologies, crystal structures, and hetero-architectures that would otherwise remain unavailable.These prospects have turned the study of nanoscale cation exchange reactions into a very relevant scientific endeavour, which has been attracting increasing interest in recent years.Cation exchange reactions are typically topotactic, thereby preserving the overall morphology and structure of the anionic sublattice of the parent nanocrystal into the product nanocrystal.There are however several recent examples of cation exchange reactions that are accompanied by morphological transformations.
In this contribution, Teranishi and coworkers report an intriguing and unprecedented example of severe morphological reorganization in response to a cation exchange rection.The authors observe that upon partial aliovalent Mn 2+ for Cu + cation exchange, hexagonal roxbyite (rb) Cu1.8S nanoplates gradually transform into crescent-shaped Cu1.8S-MnS core-shell heteronanoplates, which upon further cation exchange evolve back into complete hexagonal nanoplates of wurtzite (wz) MnS.Similar cation exchange-induced transformations are also observed with Zn 2+ , Cd 2+ , and Fe 3+ .
The work has been carried out very thoroughly and provides novel and significant insights that make it of high interest to a broad scientific community.Therefore, I think it merits publication in Nature Communications.However, there are several issues that should be addressed prior to its publication.
My main concern regards the mechanism proposed for the transformation, which is not clearly and convincingly presented in the paper.Moreover, the contextualization of the work with the most current literature in the field could be improved.I will address my concerns in more detail below.

Author reply:
We are grateful for your positive comments and constructive suggestions.Your numerous valuable suggestions encouraged us to reconsider the transformation mechanism during cation exchange (CE).Based on your comments, we proposed more detailed discussions.Q1: On page 3 (Figure 1) the authors demonstrate that under excess Mn 2+ the cation exchange reaction is clearly topotactic, maintaining the original shape and dimensions of the parent rb Cu1.8S nanoplates into the product wz MnS nanoplates (apart from a small expansion due to the slightly larger lattice volumes of wz-MnS).The authors imply in the proposed mechanism that also in this case the reaction cation exchange proceeds through a 'Waning and Waxing' cycle.However, this is not necessarily the case, given that if the CE reaction is under kinetic control, it could proceed sufficiently fast to prevent the "waning" phase.Moreover, the data presented is insufficient to support this interpretation since intermediate samples were not collected under excess Mn 2+ conditions, only under sub-stoichiometric conditions, which may favor waning for the reasons mentioned below.
Author reply: As you pointed out, Figure 1 contains the results obtained by using excess Mn 2+ .To clarify whether the CE proceeds through a 'Waning and Waxing' cycle at the ratio of MnCl2/Cu1.8S≥ 1 mol/mol, intermediate products were characterized by TEM and XRD.Since the CE reaction proceeded too rapidly to obtain intermediate products at MnCl2/Cu1.8S= 4 mol/mol, we employed the reaction conditions of MnCl2/Cu1.8S= 1 mol/mol (Fig. R6).Within the 5 min reaction, a clear 'Waning and Waxing' cycle was observed.Consequently, we would like to claim that the CE proceeds through a 'Waning and Waxing' cycle even under conditions of elevated Mn 2+ concentrations.

Q2:
The discussion concerning Figures 2 and 3 attributes the formation of the crescent-shaped Cu1.8S-MnS core-shell heteronanoplates to cation exchange.The authors suggest that Mn 2+ for Cu + cation exchange in the parent Cu1.8SNPLs begins from one side while S 2− is supplied from the opposite side through intraparticle migration.The dissolution of one half of the nanoplates is thus attributed to an imbalance between the rapid outward diffusion of Cu + and the slower inward diffusion of Mn 2+ .However, this mechanism is inconsistent with the data presented because the MnS shell is observed to grow heteroepitaxially over one half of the parent Cu1.8SNPLs, thereby making it thicker, while the other half gradually shrinks from the surface inwards.It has been shown for many different systems that core-shell heteronanocrystals formed by cation exchange reactions are characterized by superseded shell ingrowth, i.e., the total dimensions of the template nanocrystals are preserved while the shell grows thicker and the core shrinks due to the inwards diffusion flux of the guest cation and outward diffusion flux of the host cations (e.g., ZnSe-CdSe core-shell QDs [Groeneveld, E.;et al. ACS Nano (2013), 7, 7913], PbSe-CdSe core-shell QDs [Pietryga, J. M.;et al. J. Am. Chem. Soc. (2008), 130, 4879;Grodzinska, D.;et al., J. Mater. Chem. 2011, 21, 11556].The observation reported in the present contribution is reminiscent of that recently reported by Xia, C. et al. [ACS Nano 15 (2021) 9987] concerning the formation mechanism of colloidal Janus-Type Cu2-xS/CuInS2 Heteronanorods via Seeded-Injection, which is shown to start with a partial aliovalent In 3+ for Cu + cation exchange in just one facet of the Cu2-xS seed nanocrystals, converting it to wz-CuInS2.As soon as this wz-CuInS2 surface is formed, homoepitaxial growth of wz-CuInS2 takes over, outcompeting the cation exchange reaction for the limited In 3+ supply and gradually consuming the Cu2-xS segment of the nanocrystals as sacrificial Cu + sources.The formation of the Cu1.8S-MnS core-shell heteronanoplates seems to proceeds through a similar mechanism: Mn 2+ for Cu + cation exchange initially converts the bottom and top facets of the parent Cu1.8SNPLs into wz-MnS, thus triggering a fast homoepitaxial overgrowth of a MnS shell using the available Mn 2+ in solution and the other half of the NPLs as sacrificial sulfur sources.CE only resumes when the supply of S 2-has been exhausted (i.e., one half of the NPL has been entirely dissolved while the other is protected by the MnS shell) and proceeds inwards from the exposed side facets.The process would thus be triggered by a kinetic imbalance between the CE rates (too slow due to the low concentration of Mn 2+ ) and the dissolution of the Cu1.8SNPLs by Cl − and homoepitaxial growth of MnS (both fast and kinetically coupled).
Author reply: Thank you for your valuable suggestion.Firstly, no discussion regarding the relative rates of outward/inward cation diffusion was made in the above works on core-shell formation through CE reactions that you suggested.In the case of core-shell heteronanocrystals, since no extraction reagents such as TOP (as Cu + extractor) were employed, the slow outward diffusion of host cations might prevent deformation of host nanocrystals (NCs) due to the similar relative inward diffusion rate of guest cations.Additionally, shape changes are not clearly observed for small spherical NCs.In contrast, the shape changes can be clearly observed for larger plates, indicating the obvious Kirkendall-type shape transformations in several cases (e.g., Fig. R7).The conventional Kirkendall effect typically results in the formation of hollow structures for nanospheres (isotropic transformation) and rings or biconcave shapes for NPLs (isotropic transformation in in-plane direction).The Kirkendall effect should also take place at heterointerfaces of anisotropic structures due to the imbalance in atomic diffusion to achieve the anisotropic-to-anisotropic shape transformation.We believe that anisotropic shape changes induced by a significant difference in atomic diffusion rates of host and guest cations in the inplane direction are still within the scope of the Kirkendall effect.15, 9987. (ref. 36)], it is more conceivable that an initially cation exchanged position serves as the starting point for subsequent processes (further CE and MnS-shell deposition) (Fig. R8).Relatively slow inward Mn 2+ diffusion causes interparticle diffusion of ions to create structural defects on the opposite side of NPL, providing exposed fresh and unstable surface (Fig. R8b).This exposed area becomes the starting point for accelerated Cu1.8S etching by Cl − and/or TOP to release S 2-into the solution, leading to the deposition of MnS from the dissolved S 2-and Mn 2+ (Fig. R8c).Similar phenomena have been observed in the formation of hollow and pits through etching processes with Cl − , TOP and/or O2 [Y.Xiong et al., Angew. Chem. Int. Ed. 2005, 44, 7913;A. Nelson et al., Chem. Mater. 2016, 28, 8530. (ref. 35)].It is also believed that CE continues during the MnS deposition process.This is inferred from the position of Cu1.8S part in the crescent-shaped NPL, not near the edge but towards the center (Fig. R8d).

Manuscript revision:
Based on the discussion above, we revised the manuscript and added Fig. R8 as Supplementary Fig. 7 to add the MnS shell deposition processes to the transformation mechanism during waning process.
Page 10, 3rd paragraph: Based on the above characterization, we speculate the transformation mechanism of the waning process.The formation of NPLs with non-uniform thickness in the early stage (e.g., ~3 nm and ~9 nm at 10 s) from the flat Cu1.8SNPLs (5.3 nm) is likely initiated by anisotropic intraparticle ion migration within individual NPLs after the CE with Mn 2+ from one side of Cu1.8S NPLs 15,16,25 .Initiation of CE from a single location on a NC forms the starting point for the subsequent anisotropic CE, often leading to the formation of Janus-type heterostructure, as observed in many cases 8,26-34 .Explanations for this phenomenon have been often provided by the formation of a crystallographically stable heterointerface 31,33 and/or the presence of a high activation energy for the CE reaction 30,34 .These explanations would also apply to our case, where CE of the Cu1.8SNPL with Mn 2+ started from a single location.Subsequently, the imbalance between the rapid outward diffusion of the host Cu + and the slow inward diffusion of the guest Mn 2+ (as shown later) causes the anisotropic shape transformations 16 .Such a transformation triggered by a large difference in inward/outward cation migration rates has been shown in several cases, which are often explained as nanoscale Kirkendall effect 15-18 .In the case of NPLs, unique ring 15,18 and biconcave-shaped 16 nanostructure have been obtained through intraparticle ion migration in in-plane direction during CE initiated from all edges of the NPLs.In our case, the progress of CE with Mn 2+ from one side of Cu1.8S NPLs causes the directional in-plane ion migration, leading to the formation of anisotropic NPLs with non-uniform thickness.The CE continuously propagates MnS phases in NPLs from the edge, which is evidenced by the position of Cu1.8S phases within the crescent shaped NPLs, not near the edge but towards the centre (Fig. 2).
In addition to the anisotropic Kirkendall-type intraparticle ion migration, the decomposition of Cu1.8S NPLs triggered by strong coordination between Cu + and Cl − promotes the large deformation.Once partial CE with Mn 2+ occurs at a single location of NPL, interparticle diffusion of ions creates structural defects on the opposite side of NPL to provide exposed fresh and unstable Cu1.8S surface as the starting point for accelerated etching by Cl − (Supplementary Fig. 7).On such a highly reactive surface, TOP is also expected to act as a supplemental etching agent for S 2− , further accelerating NPLs dissolution 35 .The exposed Cu1.8S not covered by a MnS shell in the intermediate (observed at 10 s) is susceptible to etching and subsequently disappears (after 1 min, Fig. 3c).
Another plausible reaction in the waning process is the MnS deposition on NPLs.After the Cu1.8SNPLs is partially etched, the dissolved S 2− reacts with Mn 2+ to cause the growth of MnS on residual Cu1.8S (as shown later) 36 .Considering that the CE generally proceeds from an edge of Cu1.8S NPLs, the MnS shells on both faces of Cu1.8S NPLs might grow through this MnS deposition mechanism (Supplementary Fig. 7).These results indicate that, after the CE initiates, the kinetic balance between anisotropic CE progression, Cu1.8S etching and MnS deposition induces the specific transformation of hexagonal NPLs into crescent-shaped HNPLs, as summarized in Fig. 3n.

Q3:
On page 10 the authors state that anisotropic CE progression and Cu1.8S etching by Cl -occur simultaneously to induce the transformation of hexagonal Cu1.8S NPLs into crescent-shaped Cu1.8S-MnS HNPLs.However, as discussed above this is inconsistent with the data presented and can be better explained by a kinetic imbalance.
Author reply: As we addressed above, we added the MnS deposition process to the originally proposed mechanism.Q4: On page 11 the authors invoke the nanoscale Kirkendall effect to explain the formation of the crescent-shaped Cu1.8S-MnS HNPLs (i.e., the "waning" phase).However, this seems unlikely as the Kirkendall effect should rather result in the formation of hollow NPLs (i.e., dissolution of the center of the NPLs), as previously reported by Buhro and coworkers for CuInS2 NCs (ref. [18]).To support this interpretation, the authors should also propose a reasonable explanation for the asymmetry observed in the process: why does one half of the NPL dissolves, while a MnS shell overgrows on the other?It seems more likely to me that the dissolution is driven by a kinetic imbalance in between etching rates by Cl -and the CE rates (similarly to the formation of hollow Cu2-xS nanocrystals through etching induced by GaCl3 [Hinterding, et al.;ACS Nano (2019), 13, 12880]).
Author reply: As we addressed above, the combination of CE, Kirkendall-type deformation and etching of Cu1.8S plays an important role in the revised mechanism of anisotropic shape changes in this study.A decisive factor in determining the final shape depends on where the CE initiates.Initiation of CE from a single location on a NC forms the starting point for the subsequent CE, often leading to the formation of Janus-type heterostructure, as observed in many cases [refs. 8, 26-34].Explanations for this phenomenon have been often provided by the formation of a crystallographically stable heterointerface and/or the presence of a high activation energy for the CE reaction.These explanations would also apply to our case, where CE of the Cu1.8SNPL with Mn 2+ started from a single location, and subsequently an unstable surface is exposed on the opposite side as a result of deformation due to the imbalance in cation diffusion rates.This exposure accelerates etching at that side, resulting in the formation of the crescent-shaped plate.Although we do not have clear answer to the question why the CE reaction starts at a single position of NPL, as we showed in the previous paper (Science 2021, 373, 332.), the position where the CE reaction starts is greatly dependent on the NC shape.

Q5:
The authors convincingly demonstrate that chloride is responsible for the etching and dissolution observed during the waning phase.However, metal chlorides are widely used to deliver guest cations in cation exchange reactions and do not seem to induce any etching.Why do they act as etching agents under the conditions used in the present work?What is different with respect to the conditions typically used in other works?
Author reply: As you pointed out, metal chloride is widely used in many CE studies, and we can rarely find the severe etching and dissolution.In numerous cases, the NCs are inherently surrounded by thermodynamically stable crystal facets that are effectively protected by ligands, preventing Cl − and/or TOP from attacking.However, if outward/inward cation diffusion is imbalanced, it causes a change in NCs' shape to destabilize stable facets.Consequently, the exposed unstable surfaces are preferentially attacked and etched by Cl − and/or TOP.Since partial CE reactions of NPLs or nanorods with Mn 2+ are quite rare, our results seem to be specific to the NC shape.Because we can find few reports on the details of size and volume changes NCs after partial CE reactions, the effect of etching observed in this study might have been overlooked.

Q6:
The discussion on page 11 and elsewhere in the paper overlooks the importance of the solidstate diffusion rates of the guest cations in determining the overall heteroarchitecture of the product nanocrystals obtained by cation exchange reactions.This is a crucial point because it depends on the nature of both the guest cation and the host nanocrystal and on the temperature.Slower migration of the incoming guest cation does not necessarily result in collapse of the anionic sublattice and dissolution of the NC, but rather in the formation of heteronanocrystals with welldefined heterointerfaces, while fast diffusion results in homogeneous alloys.This has been extensively investigated by many groups, e.g.,      JACS 2016, 138, 7082. (ref. 31)] into a smaller number (area) of heterointerfaces (Fig. R13).These works suggest that a more crystallographically stable Cu1.8S-MnS heterointerface is spontaneously formed during the CE progression in our case.Zhou et al.

Manuscript revision:
To clearly explain the transformation mechanism of waxing process to readers, the above discussion on the waxing process has been partly added to the main text.Figures R10 and R12 were added as Supplementary Fig. 14 and 15, respectively.Page 14, 2nd paragraph: Because the transformation from the crescent-shaped to the intermediate NCs with bilayer structure (at 30 sec) involves a large structural change, there should be other intermediate structures in the earlier stage.However, the waxing stage progresses quite rapidly, making it difficult to experimentally capture fine intermediate snapshots.In previous works, reconstruction of heterointerfaces in NCs has been often observed in partial CE reactions.The large mobility of cations in Cu1.8S especially under heated conditions can rearrange two distinct domains with multiple patchy structure 40 or core-shell structure 31 into a smaller number (area) of heterointerface.These works suggest that, in our case, the Cu1.8S-MnS bilayer structure is spontaneously formed by generating more thermodynamically stable heterointerface from the Cu1.8S@MnS core@shell structure during CE progression 41 .Page 15, 3rd paragraph: A distinctive phenomenon in the waxing process is the decrease in the NPL thickness when the Cu1.8S-MnS bilayer structure (7.5 nm at 10 s) is transformed into MnS NPL (5.2 nm).We suggest two possible scenarios of how the thickness of the NPL reverts back to the original thickness.The first is based on the reconstruction process after the CE reaction.If the CE proceeds in the Cu1.8S layer of Cu1.8S-MnS bilayer structure, a thick MnS plate will form exclusively in that region, resulting in the formation of MnS NPLs with uneven thickness (Supplementary Fig. 14a).Considering the formation of flat, thin MnS NPLs, the shape reconstruction should take place to make the thickness uniform after the CE.Completely flat NPLs seem more stable due to the reduced surface energy than those with uneven thickness.In another scenario based on the etching and deposition process, the Cu1.8S layer of Cu1.8S-MnS bilayer structure is rapidly etched by Cl − and/or TOP, leaving a thin MnS layer and releasing S 2− as the precursor for MnS growth (Supplementary Fig. 14b).Because the thickness of MnS parts in bilayer structure (10 s in Fig. 5) is slightly thinner (~4.7 nm) than that of final MnS NPLs (5.2 nm) (Supplementary Fig. 15), the two scenarios may occur simultaneously rather than just one process or the other.

Q9:
In the final section of the paper the authors discuss their experiments with nanorods.How do their observations differ from what has already been extensively reported by Schaak and coworkers? (e.g.,) Author reply: Our experiments with nanorods were conducted to confirm the versatility of the waning-and-waxing process in the CE reactions.In other studies on partial CE of nanorods using ZnCl2 precursor (refs. 32 and 42) the CE reactions were performed at 50°C and 90°C, respectively, which are lower than in our study, suggesting that the etching process may not be activated (since higher reaction temperatures accelerate the etching of Cu1.8S NCs [G. A. Di Domizio et al., Chem. Mater. 2021, 33, 3936. (ref. 37)]).Another noticeable aspect in comparison with other studies is the detailed investigation of the changes in width, length, and volume of partially cation-exchanged nanorods (as well as NPLs).Structural changes beyond lattice volume changes were found through such detailed investigation, and also the novel phenomena such as partial etching were observed, which have been overlooked.Q10: Claim of universality at the end of paper is likely exaggerated.The authors investigated four different cations: Mn 2+ , Zn 2+ , Cd 2+ and Fe 3+ .The waning-waxing cycle process occurs only for Mn 2+ and Zn 2+ .Cd 2+ gives Janus-type hetero-NPLs and Fe 3+ gives homogeneous CuFeS2.What determines the different outcomes?Likely the different solid-state diffusion rates of the different cations, but this is an aspect that is largely overlooked in the mechanism proposed in the paper.
Author reply: As you pointed out, the waning process was observed only for Mn 2+ and Zn 2+ , limiting the range of applicable metal cations in our experimental conditions.In the experiments in Fig. 6, partial CE of Cu1.8S NPLs was conducted with Mn 2+ or Zn 2+ at first, and the subsequent CE reactions of chipped Cu1.8S-(MnS or ZnS) HNPLs were performed with four different cations (waxing process).We expected that the waxing process might occur even with different metal cations based on the assumption that the waxing process involves both the CE of the remaining Cu1.8S and the deposition of metal sulfide formed from supplied metal cations and dissolved S 2− , as observed in the case of Mn.As a result, the overall hexagonal-plate shapes are restored with any metal cations (Fig. 6), suggesting the potential use of various metal cations in the waxing process.We think that the different outcomes are probably derived from the thermodynamic stability of the resulting crystal phases (CuFeS2 phase is formed easier than Fe2S3 phase).Therefore, we believe that the waxing process is general and versatile.

Manuscript revision:
To precisely conclude the manuscript, we revised the summary as follows.
Page 19, 2nd paragraph: We have also shown that the waning and waxing strategy can be induced by other metal cations and can be applied to NRs and that the waxing cycle is a general and versatile process.

Figure R1 .
Figure R1.EDX results of three solids containing Cu, Mn, and S with controlled mole ratios.Gray and bars stand for calculated and experimental EDX values, respectively.Five distinct areas were measured for EDX analysis to obtain the average values and standard deviations.

Figure R2 .
Figure R2.Representative raw EDX spectra of Cu1.8S NPLs, Cu1.8S-MnS heterostructured NPLs (HNPLs) and MnS NPLs.Green regions show background intensity.Signals from C, O and Al were detected from carbon tape substrates (C and O) and sample stages (Al).Cu-K, Mn-K and S-K lines are used to quantify mole ratios.

Figure R3 .
Figure R3.EDX results of three solids containing Cu, Mn, and S with controlled mole ratios.Gray and bars stand for calculated and experimental EDX values, respectively.Five distinct areas were measured for EDX analysis to obtain the average values and standard deviations.

Figure R5 .
Figure R5.TEM images of product NPLs: (a-c) products of entry 1-3 listed in Table R5; (d-e) products shown in Extended Fig. 1a in the manuscript.Mn/Cu values represent the Mn to Cu mole ratio from EDX measurements.Scale bars are 100 nm.Arrows in (b) indicate anisotropic dissolution of NPLs.

Figure R8 .
Figure R8.Schematic of the formation of crescent-shaped Cu1.8S/MnS heterostructured NPL (HNPL) including CE and MnS deposition mechanisms from the side view.

Figure R10 .
Figure R10.Schematic of conversion of Cu1.8S into MnS in the final stage of CE from the side view: (a) homogenizing of uneven thickness after CE of Cu1.8S layer; (b) repairing damaged part with MnS after etching of Cu1.8S layer.

Figure R12 .
Figure R12.HAADF-STEM image of Cu1.8S-MnS HNPLs at 10 s in the waxing process with total and MnS layer thicknesses.